Part 2 - Food Chains and Webs:Part 2 - Food Chains and Webs:

• If an ecosystem is to be self-sustaining it must contain a flow of energy.

• Those life activities that are characteristic of living organisms require an expenditure of energy.

• The pathways of energy through the living components of an ecosystem are represented by food chains and food webs.

• Producers convert the radiant energy of the sun into the chemical energy of food.

A. Food chain: involves the transfer of energy from green plants through a series of organisms with repeated stages of eating and being eaten

B. Food web: In a natural community, the flow of energy and materials is much more complicated than illustrated by any one food chain.

• Since practically all organisms may be consumed by more than one species, many interactions occur along the food chains of any community.

Food Web Interactions:Food Web Interactions:

1. Producers: (plants) -- the energy of the community is derived from the organic compounds in plants

- (grass in the web above)

2. Primary Consumer: (always a herbivore)

- feeds on plants (mice, grasshoppers, and rabbits in the web above)

3. Secondary Consumer: (always a carnivore) -- feeds upon other consumers (frogs, sparrows, snakes, and foxes above) (The hawk is a secondary or 3rd level consumer depending on the availability of

food.)

Omnivores may be primary or secondary consumers.

4. Decomposers: break down organic wastes and dead organisms to simpler substances (ex. bacteria of decay)

** Through decomposition, chemical substances are returned to the environment where they can be used by other living organisms.

Energy Flow:Energy Flow:

• Energy flows through ecosystems in one direction, typically from the Sun, through photosynthetic organisms, including green plants and algae, through herbivores, to carnivores, and finally decomposers.

• There is a decrease in the overall energy in each level as you move up the food web.

• This means that there is much more energy in the producer level in a food web than at the consumer levels.

• Also, this means that there is more energy at the primary consumer level than at the secondary consumer level.